The digital phosphor oscilloscope architecture combines the advantages of analog and digital oscilloscopes

This is because both platforms have their own advantages. DSOs also provide multi-channel operation as well as measurement automation and waveform storage capabilities.

The grayscale display, variable brightness, and continuous acquisition capabilities of analog oscilloscopes inherently bring real-time "statistics" to the waveform being viewed. It highlights the most frequently occurring portions of the signal, but analog oscilloscopes lack the storage capacity and other features of DSOs. Engineers have not been able to rely solely on one oscilloscope architecture to meet all signal characterization needs.

DPO: A major breakthrough in structure

Tektronix The new oscilloscope platform, the Digital Phosphor Oscilloscope (DPO), combines the best of both worlds and goes beyond both technologies. Engineers can now use a single instrument to capture all the important information about a waveform: amplitude, frequency, and an intensity axis that reveals how the amplitude was distributed during the measurement.

The digital phosphor oscilloscope is as great a measurement technology breakthrough as any. It offers all the advantages of the DSO architecture, from digital storage to sophisticated triggering capabilities. It meets the need for analog-style features, such as grayscale display and real-time operation, by digitally emulating the chemical phosphor process that produces intensity gradients in the analog oscilloscope CRT. It transforms the digital oscilloscope into a general-purpose waveform acquisition instrument.

Figure 1. Simplified block diagram of a DPO system.

Tektronix has leveraged its experience in DSO advanced technologies such as digital real-time acquisition and InstaVuTM acquisition to create remarkable performance for DPO. Figure 1 is a simplified block diagram of a DPO system.

A digital phosphor oscilloscope can continuously acquire waveforms into a three-dimensional database while updating the display because it uses a parallel processing architecture that integrates the display and acquisition system. Note that display management tasks do not burden the DPO's system processor. The processor is dedicated to measurement automation and analysis. This is in contrast to a typical DSO, where every bit of data that reaches the display must pass through the processor, which is also responsible for performing calculations, managing the oscilloscope's user interface, and so on.

Figure 2. DPO waveform image showing how the trace intensity reveals the frequency of occurrence.

At the heart of the DPO is the DPX™ waveform image processor, a proprietary ASIC that rasterizes digitized waveforms into a dynamic, three-dimensional database called a digital phosphor oscilloscope. DPX accumulates signal information in a 500 x 200 array of integers. Each integer in the array represents a pixel in the DPO display. If a signal flows through a point over and over again, its array position is repeatedly updated to emphasize this occurrence. Over a time span consisting of many sample points, the array builds a detailed picture of the signal's integrity. The result is a waveform trace whose intensity varies in proportion to the frequency of the signal occurring at each point, a "grayscale" just like an analog real-time oscilloscope. But unlike an ART, a DPO allows the use of color to express gray levels. Figure 2 shows this effect using a waveform from a metastable circuit. The intensity level explicitly indicates how often each point on the screen occurs. A histogram above the main trace statistically represents the intensity information in the trace itself.

Figure 3. A) The analog oscilloscope is the recognized waveform profile;

Figure 3.B) Capturing the entire signal envelope requires a lower sampling rate, which results in artifacts that distort the DSO display of the video signal;

Figure 3. C) The DPO shows a video waveform without aliasing; many parts of the waveform are emphasized, indicating that the signal spends more time at those points.

The DPO operating model relies on its outstanding display sample density to show signal operation in real time. Traditional DSOs only sample for a short period of time, which accounts for less than 1% of the total time. The rest of the time is spent compiling display windows, which occasionally ignores all signal operations occurring at the time. In contrast, DPOs utilize Tektronix's proven signal capture patented technology - InstaVu acquisition technology, which significantly reduces the idle time between acquisitions. DPOs can record up to 200,000 waveforms per second, which is 1000 times more signal data than ordinary DSOs. New digital phosphor snapshots are sent to the display every 1/30 second without interrupting the acquisition process. As a result, the image corresponds to the waveform operation in real time, and the rich data can accurately represent the waveform.

A "persistence" mode is sometimes used in DSOs to simulate grayscale. However, a persistence display is created by post-processing a normally acquired waveform, not a real-time acquired waveform. Persistence relies on many accumulated "screens" of data, requiring time to reacquire and recalculate the display results. A DPO, on the other hand, integrates the display and acquisition systems to display three-dimensional signal information in real time, which can be viewed immediately on the screen.

Using DPO in a real-world setting

We have seen that both ART and DSO have their own advantages and disadvantages. For the first time, DPO offers a platform that combines all the advantages without their disadvantages and surpasses both devices.

The best way to verify this is to look at some real-world measurement examples.

Figure 4. QAM constellation diagram as seen on a Tektronix DPO screen. The DPO continuous acquisition feature provides a dynamic, accurate XY display.

Figure 5. Aberrations showing blurring of the central pulse occur less frequently than the normal pulse shape. This visible phenomenon quickly reveals irregular transients.

Figure 6. A three-dimensional view of waveform data from a digital phosphor array. The frequency of occurrence is represented by the Z-axis of the graph.

A solution for capturing signals with long time intervals. Packetized signals consisting of multiple components with relatively long time periods are difficult to capture intact using a DSO. An example is the composite video signal in Figure 3a. It requires capturing a long time interval (thus using a slow time base setting) to understand the characteristics of the entire envelope. However, in order to capture the details of the individual pulses, a fast time base setting must be used.

Normal procedure is to set the DSO's timebase (and therefore its sampling rate) to a slow enough horizontal rate to capture the entire signal envelope. On a DSO, a slow sampling rate will produce artifacts on the individual faster pulses within the signal - a byproduct of using a sampling rate that is too slow relative to the frequency being measured. The result is a waveform that is distorted to the point of being almost unrecognizable, as shown in Figure 3b. Even worse, the frequency of the waveform will appear to be lower than its actual frequency.

Until now, the solution has been to use an analog oscilloscope to view such signals. The analog display in Figure 3a is considered the "correct" waveform profile. However, ART cannot store, automatically measure and analyze signals.

The DPO provides ample waveform data, delivering 100 million samples per second to the display, eliminating the aliasing problem. The resulting waveform (Figure 3c) is clear and comprehensive, even though it was acquired on a slow timebase setting. Note that in Figure 3c, many of the waveform portions are emphasized, indicating that the signal spends more time at those points. This axis of information is completely missing from the real-time DSO display. Also note that the signal shown is a stable test pattern. If it were a dynamically changing live video signal, the DSO display would be further off the mark.

Glitch has plagued DSOs for years. Video measurements, disk drive read channel measurements, wireless communications signals, and other measurements that require capturing long "packets" of fast pulses have kept engineers clinging to ART oscilloscopes. The advent of the Tektronix DPO finally solves the glitch effect of digital oscilloscopes.

The end result: a digital oscilloscope with true XY mode. There is no substitute for true XY measurement capability in an oscilloscope. In XY mode, you compare the phase relationship of two signals by feeding one signal into the normal vertical input and the other into the horizontal input. XY mode is a traditional advantage of analog oscilloscopes, but it also has real-time data throughput requirements, which is a disadvantage of DSOs.

However, the complex digitally modulated signals in current wireless communications require digital oscilloscopes to provide additional functions, such as bandwidth, triggering, analysis, and so on.

Figure 4 is a QAM constellation diagram captured by a Tektronix DPO. The earlobes depicting the 90-degree phase shift point are clear and stable. This is because the DPO continuously extracts samples at a rate of 10.4M samples/second into the digital phosphor, which continuously scans this information onto the display at a rate of 1M pixels/second. This continuous acquisition capability provides a dynamic, accurate XY display.

A DSO cannot produce such a display. A DSO does not provide sufficient sample density or continuous acquisition capability. In addition, the color gradient of a DPO provides better resolution than the monochrome grayscale of an ART.

Revealing random and infrequent events. The DPO's ability to capture random or infrequent events is particularly useful for debugging state-of-the-art electronic designs. Here again, the DPO's superior display sample density means the oscilloscope spends much more time actively acquiring data rather than processing it for display. This means that the occasional transient event is much less likely to be overlooked. In addition, the grayscale feature highlights the frequency of these transient signals relative to other signal components on the screen.

Figure 5 shows a signal composed of widely separated pulses as well as intermittent noise and transient events. Note the blurred aberration in the center pulse shown, which is a variation of the pulse that occurs less frequently than the normal pulse shape. The ability to detect this aberration is particularly useful in troubleshooting applications.

A whole new level of data analysis. Because the DPO stores waveform data in a dynamic, three-dimensional database, statistical information about the signal can be easily derived. The DPO's internal histogram function collects quantitative information about the signal distribution in transit or stored waveforms. The three-dimensional database can also be exported to an external PC through the oscilloscope's GPIB port for analysis, including three-dimensional plotting. The data provides a three-dimensional view where the frequency of occurrence is represented by the Z axis of the graph. As with the DPO screen display, color can be used to enhance the display. Figure 6 shows the resulting graph.

in conclusion

The new digital phosphor oscilloscope from Tektronix combines the advantages of both analog and digital oscilloscopes in one powerful acquisition technology. This measurement tool is superior to both analog and digital oscilloscopes because it can examine signal operation in ways never before possible. The capabilities of a digital phosphor oscilloscope cannot be achieved with any existing oscilloscope architecture, whether analog or digital.